The so-called “general-purpose plastics” such as polyethylene (hereinafter, referred to as “PE”), polypropylene (hereinafter, referred to as “PP”), polystyrene (hereinafter, referred to as “PS”), and polyvinyl chloride (hereinafter, referred to as “PVC”), are commonly used as materials for various daily-use products (such as bags, various wrappings, various containers, and sheets) and materials for industrial parts of automobiles and electrical products, daily necessities, miscellaneous goods, and the like, not only because they are available at very low prices of 100 yen or less per kilogram, but also because they are easy to mold and are lighter in weight than that of metal and ceramics (are a fraction of the weight of metal or ceramics).
However, the general-purpose plastics have drawbacks such as that the general-purpose plastics are insufficient in mechanical strength and that they have low heat tolerance. Accordingly, the general-purpose plastics currently are limited in its applicable range, since the general-purpose plastics do not fulfill the sufficient properties required as materials used for various industrial products, e.g. mechanical products such as automobiles, and electrical, electronic, and information products. For example, PE typically softens at a temperature of approximately 90° C. Further, PP, which is considered to have a relatively high heat tolerance, typically softens at approximately 130° C. Moreover, since PP is insufficient in transparency in comparison with polycarbonate (hereinafter, referred to as “PC”), polyethylene terephthalate (hereinafter, referred to as “PET”) and PS, PP suffers from such a drawback that it cannot be used as optical materials, bottles, or transparent containers.
On the other hand, the so-called “engineering plastics” such as PET, PC, fluoroplastics (e.g. Teflon (registered trademark)), nylon, polymethylpentane, polyoxymethylene, and acrylic resin, have excellent mechanical strength, heat tolerance, transparency, and like properties, and typically do not soften at 150° C. Therefore, the engineering plastics are used as various materials for industrial products such as automobiles, mechanical products and electric products which require high performance, and optical materials. However, the engineering plastics suffer from serious drawbacks: For example, the engineering plastics are expensive, and the engineering plastics are very environmentally unfriendly because it is difficult or impossible to convert them back into monomers for recycling.
Therefore, if the material properties such as mechanical strength, heat tolerance, and transparency of the general-purpose plastics are so remarkably improved that the general-purpose plastics can replace the engineering plastics and even metal materials, it becomes possible to greatly reduce costs of various industrial products and daily-use products made of polymers and metals, greatly save energy through a reduction in weight, and improve its operability. For example, if PP can be used instead of PET which is currently used as bottles for beverages such as soft drinks, this allows for greatly reducing the costs of bottles. Although it is possible to recycle PET into monomers, it is not easy to carry this out. Hence, used PET bottles are cut, are reused once or twice in low-quality applications such as using as clothing fibers and films, and thereafter are discarded. Meanwhile, PP can be easily recycled into monomers; this allows a complete recycling of PP, thus bringing about a merit that it is possible to reduce the consumption of fossil fuels such as oil and reduce generation of carbon dioxide (CO2).
As mentioned above, in order to improve the properties such as the mechanical strength, heat tolerance, and transparency of the general-purpose plastics to use the general-purpose plastics as a replacement of the engineering plastics and metals, a remarkable increase is necessary in the proportion of crystals (crystallinity) in PP or PE, or more preferably, a crystal substance which is purely crystalline and which hardly contains an amorphous PP or PE is necessarily prepared. Particularly, high expectations are placed on PP, since PP is advantageous in that it has a stronger mechanical strength and a higher heat tolerance as compared to PE. Further, PP is an important polymer which maintains a high yearly production increase rate of several percent.
One method known to improve crystal properties of a polymer is to cool melt of the polymer at a slow rate. This method, however, is totally insufficient in the increase of crystallinity. Further, this method causes a significant deterioration in productivity of products, and further causes an increase in crystal grain size to a bulky size, thus causing a decrease in mechanical strength. Another method proposed to increase the crystallinity is to cool the melt of the polymer under high pressure. This method, however, requires applying a pressure of several hundred atm or greater to the melt of the polymer. Although this method is possible theoretically, it is not feasible in industrial production due to the complicated design required of the production apparatus and due to its high production cost. Thus, this method is difficult to accomplish practically. Another method known to improve the crystal properties of the polymer is to add a nucleating agent to the polymer melt. However, this method currently suffers from the following drawbacks: (a) inevitable contamination of the nucleating agent as impurities, and (b) an insufficient increase in crystallinity, and an increase in cost due to the nucleating agent being much higher in cost than that of the resin. In conclusion, there is currently no complete method to dramatically improve the crystallinity of a polymer such as the general-purpose plastics, and to produce a crystal substance of the polymer.
Incidentally, many studies have shown that the polymer melt (isotropic melt) in which molecular chains take random conformation (e.g. “random coil”) is crystallized under shear flow to sparsely generate a combination of shish crystal form and kebab crystal form in the polymer melt (see Non patent Literature 1). The shish crystal form is a fiber-like crystal of several μm in diameter and is oriented along the flow. The kebab crystal form is a lamination of thin-film crystal and amorphous skewered through the shish crystal form. This form is referred to as “shish-kebab”, meaning “skewer” and “meat” of skewered grilled-chicken (Japanese “Yakitori”).
In the production of the shish-kebab form, only the shish form is created locally in an initial period. The shish form is of an Extended Chain Crystal (ECC) structure in which straightly-elongated molecular chains are crystallized (see Non patent Literature 5). On the other hand, the crystal portion of the kebab form is of a Folded Chain Crystal (FCC) structure in which the molecular chains are folded at a surface of the thin-film crystal. How the shish-kebab form is produced has not been explained in terms of molecular theory, since no studies have been carried out kinetically, and thus was unknown. The FCC is a thin-film crystal (called a lamellar crystal) which is most widely seen among polymer crystals. Moreover, it is commonly known that injection molding forms a “skin” (which is a thin crystalline film of several hundred μm thickness) on surface, and a “core” inside. The core is an aggregate of “laminated structures (laminated lamellar structures)” in which the folded chain crystal and amorphous are laminated (see Non patent Literature 6). It is considered that the skin is formed from the shish-kebab form, but the shish has been observed as being formed only sparsely. No studies have been performed based on kinetic study on the production mechanism of the skin structure, and hence the production mechanism remains totally unknown.
The inventors of the present invention are pioneers to study the production mechanism of the shish form kinetically, and found the mechanism of the local formation of the shish form in the melt: at a boundary with heterogeneity, some molecular chains in the melt attain liquid crystal orientation because the molecular chains are elongated due to “topological interaction” with the boundary, and the melt becomes “Oriented melt” (e.g., see Non patent Literatures 2 and 3). Here, the “topological interaction” is an effect of “string-like polymer chains pulling each other because the polymer chains are entangled”. The topological interaction is well known as an interaction unique of the polymer. The inventors of the present invention are first to report a theory of the topological crystallization mechanism of polymers, explaining how the ECC and FCC are formed. This theory is called “sliding diffusion theory” and is recognized worldwide (see Non patent Literature 7).
Moreover, the inventors of the present invention reports, through an elucidation of a generation mechanism of “spiralite” found in a shear flow crystallization at a low shear strain rate of 0.01 to 0.1 s−1, a general mechanism that in shear crystallization, a shear strain rate of polymer melt remarkably increases at an interface of solid and liquid phases, which causes an increase in an elongate strain rate, and by this increase, the molecular chains are elongated to locally form the oriented melt, thereby remarkably speeding up nucleation and growth speed (see Non patent Literature 4).
Based on these, it is expected that the polymer crystallization will be facilitated and high crystallinity can be achieved if the entire polymer melt becomes the oriented melt by applying a large elongation strain rate which exceeds the “critical” elongation strain rate (called critical elongation strain rate) of the polymer melt. The polymer melt which has entirely become the oriented melt is referred to as “bulk oriented melt”. Further it is expected that if the bulk oriented melt can be crystallized with the orientation, a crystal structure in which a majority of the molecular chains of the polymer are oriented can be produced (the crystal structure is referred to as bulk “polymer oriented crystals”). In this case, the nucleation is significantly facilitated and a vast number of nuclei are generated between molecular chains without adding a nucleating agent thereto. This eliminates the need of the addition of the impurity and allows a crystal size to be in nanometer order. It is expected that this leads to obtaining polymers with high transparency and with a dramatically improved mechanical strength and heat tolerance.
The inventors of the present invention carried out continuous studies to provide a method of producing polymer crystals having excellent properties in properties such as mechanical strength, heat tolerance, and transparency, and to provide polymer crystals produced by such a production method. As a result, they found that polymer crystals having the excellent properties are achievable by elongating melt of a polymer (also called “polymer melt”) at an elongation strain rate not slower than a critical elongation strain rate, to make the polymer melt into an oriented melt, and thereafter cooling the oriented melt in that state for crystallization (for example, see Patent Literatures 1 and 2).